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Clinical Optics and Dispensing > Week 6: Prism, Bifocals & Progressives > Flashcards

Week 6: Prism, Bifocals & Progressives Flashcards

1
Q

One eye may be looking through the distance portion of a bifocal, while other eye is looking through near zone. How would you resolve this issue?

A

Seg height can be raised up or down to ensure both eyes reach the seg at the same time when rotating (looking) downwards

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2
Q

What should you do when prescribing vertical prism for bifocals?

A

If prescribing grind prism, eye will deviate to look towards apex
- Fitting cross should be raised 0.3mm for every dioptre of base down prism
- Lowered 0.3mm for every dioptre of base up prism required

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3
Q

Issue: Prescribing prism at near for bifocals, but not for distance. How would you resolve this issue?

A
  • Prism at near can be achieved by decentering the near segments
  • When decentering near segments, ensure reading zone area is large enough to facilitate reading
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4
Q

How to prescribe prism slab-off/bi-prism bifocals

A
  • Incorporate slab-off (i.e. remove base down) into the lens for the eye which needs to be more base up relative to the other eye
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5
Q

What type of lenses can be used for prism slab off?

A
  • Are typically only used in bifocal types which have a straight dividing line between distance & near segments
  • E.g. flat-top bifocals (a) and executive bifocals (b)
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6
Q

What is cemented or bonded segments

A
  • These can incorporate any kind of prism or prescription modification in the distance & near portions
  • Shape, size & location can be manipulated independently
  • Can be used to control differential prism
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7
Q

What is a progressive addition lens?

A
  • A lens having optics that vary in power such that the power gradually increases from the distance, to near portions of the lens
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8
Q

How are Progressive Lens Produced?

A
  • Lens is produced such that the curvature of the front surface is gradually increased as we move further down the lens
  • Process is known as optical modulation
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9
Q

What is the ‘add’ in progressive lenses?

A
  • A PAL would permit clear vision at any given distance (up to the set reading distance for that patient)
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10
Q

List the different zones of a PAL

A
  1. Distance Portion/Zone
  2. Progressive Corridor/Zone
  3. Near Portion/Zone
  4. Periphery
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11
Q

What is Distance Portion/Zone of a progressive?

A
  • Largest section of the PAL lens, will vary in size depending on the design of the lens
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12
Q

What is Progressive Corridor/Zone of a progressive?

A
  • Area of gradually increasing power allowing vision at intermediate distances
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13
Q

What is Near Portion/Zone of a progressive?

A

Restricted area of clear near vision
- All of the same power (the distance + add)

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14
Q

What is Periphery of a progressive?

A
  • Side portions which contains unwanted astigmatism
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15
Q

What is unwanted cylinder/distortion in PALs?

A
  • Due to the design of the PAL, this distortion cannot be avoided - it must be managed
  • The distortion varies in its amount & orientation depending on the design and prescribed add of the lens
  • Amount of distortion increases with an increase in the add prescribed
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16
Q

What are some likely symptoms when one is looking through the periphery of a PAL?

A
  • Distortion
  • Blur
  • Headaches
  • Swimming effect
  • Eyestrain
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17
Q

List 3 advantages and disadvantages of a PAL

A

Advantages:
1. Cosmesis
2. All distances are clear in one pair of specs
3. Can be cheaper than separate pairs

Disadvantages:
1. Lenses are expensive
2. Distortion through lenses
3. Takes time to adapt to lenses
4. Requires careful frame selection

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18
Q

Who Should Wear PALs (list 4)

A
  1. Presbyopes
  2. Esophoric myopes whose script is progressing
  3. Accommodative strabismus
  4. Pre-presbyopes with accommodative problems
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19
Q

Who should not wear PALs (list 4)

A
  1. Satisfied bifocal wearers
  2. Limited finances
  3. Monocular patients
  4. Low myopes
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20
Q

Where does Lowest Add power occur?

A

At distance reference point (DRP)

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21
Q

Where is Highest Add power located?

A

At near reference point (NRP)

22
Q

List the two different corridor lengths and describe briefly

A
  • Long corridor length = softer design & easier to adapt to
  • Short corridor length = enable the wearer to reach an adequate near vision zone, particularly in a shallow eye frame
23
Q

What are the advantages & disadvantages of short corridors in PALs?

A

Advantages:
1. More near utility in smaller frames
2. Reduced eye declination during near vision

Disadvantages:
1. Less intermediate vision and mid-range utility
2. More rapidly increasing unwanted astigmatism
3. Distortions at bottom of the lens if the frame is too deep

24
Q

What does the marking and engravings represent in PALs?

A
  • Locating correct areas for fitting the lens & for power measurements
  • Indicating the add of the lens
  • Indicating the brand and design of the PAL
25
Q

List what is on the permanent engravings on PALs

A
  1. Lens reference points
  2. Add
  3. Near Portion/Zone
  4. Lens Identification logo
26
Q

What is the ‘Lens reference points’ in progressive lenses?

A

Indicate the horizontal meridian of the lens, and as a guide to place further markings on the lens

27
Q

Where is the ‘add’ located in progressives?

A

Always located temporally on the lens

28
Q

What is the ‘Near Portion/Zone’ in progressives?

A

Restricted area of clear near vision

29
Q

What is the ‘Lens Identification logo’ in progressives?

A

Listing the lens identification logos so can identify the brand & design of the lens

30
Q

What is the ‘Lens Identification logo’ in progressives?

A

Listing the lens identification logos so can identify the brand & design of the lens

31
Q

List the lens markings for PALs

A
  1. Distance checking zone
  2. Near checking zone
  3. Fitting cross
  4. Major reference point (MRP)
32
Q

What is the ‘distance checking zone’ in progressives?

A

The distance prescription of the lens is checked through this zone using a focimeter

33
Q

What is the ‘near checking zone’ in progressives?

A

Near prescription can be read through this zone

34
Q

What is the ‘fitting cross’ in progressives?

A

The lens should be fitted such that the fitting cross sits in front of the centre of the pupil

35
Q

What is the ‘major reference point (MRP)’ in progressives & where is it located?

A
  • Any prism contained in the lens is measured at this point
  • Located at the beginning of the progressive corridor
36
Q

What is prism thinning?

A
  • PAL’s can become quite thick due to the increasing curvature in the lower portion of the lens
  • Prism thinning is used in order to make the lenses thinner & lighter
  • Amount of base down prism applied varies with the add of the PAL
37
Q

What are the different PAL designs?

A
  1. Hard vs Soft designs
  2. Monodesign vs Multidesign
  3. Customised designs
  4. Freeform designs
38
Q

What is hard design for PAL?

A
  • Contains large distance & near zones, however unwanted cyl is highly concentrated in certain areas, making adaptation to the lens harder
  • The power in the progressive zone tends to increase faster, meaning the zone is shorter
39
Q

What are the advantages & disadvantages of hard design in PAL?

A

Advantages:
1. Can exclude cyl from certain areas
2. Larger delineated distance and reading zones
3. Shorter progressive channel

Disadvantages:
1. Areas of concentrated distortion
2. Longer and more difficult adaptation
3. Progressive zone shorter

40
Q

What is soft design for PAL?

A
  • Has smaller distance & near zones, however the transition from near to the periphery is much more gradual
  • Smaller amounts of cyl are spread out over a much larger area
41
Q

Advantages & disadvantages of soft design in PAL?

A

Advantages:
1. Easier, more rapid adaptation
2. Less distortion of peripherally viewed objects
3. Less ‘swim’ of objects with head movement
4. Dioptric power of unwanted cyl is lower

Disadvantages:
1. Possible slightly compromised distance clarity
2. Longer progressive channel

42
Q

What is the Monodesign Lens in PALs?

A
  • Chooses a basic design that is used for all add powers offered
43
Q

What is the Multidesign Lens in PALs?

A
  • For a given type of PAL, a different design is used depending on the add of the lens
44
Q

Describe aspherisation in PALs

A
  • Allows for flatter lens designs without increasing aberrations (notably oblique astigmatism)
  • Improved cosmetics with better quality vision
  • Traditional methods for measuring lens power will have shortcomings (i.e. focimetry) will have shortcomings, which require greater trust in the lens manufacturers
45
Q

Describe Hoyalux iD (Physiological Optics)

A
  • Front surface has vertical progression
  • Back surface has horizontal progression
46
Q

List the different Varilux Physio (Wavefront Analysis)

A
  1. Wavefront aberrometry
  2. High order aberrations
  3. Digital direct surfacing
47
Q

Describe Wavefront Aberrometry briefly

A
  • Analyses aberrations generated by the reference spec lens surface
48
Q

Describe Digital Direct Surfacing (on back surface)

A
  • Takes into account the power created by the front & back lens surfaces
  • Back surface is calculated point-by-point to maintain the wavefront created by the front lens surface & creates a “best form Rx” for each wearer
49
Q

What is extended focus lenses in PAL?

A
  • Have a large field of view for reading, with a restricted field for either the distance, or intermediate vision (opposite to standard PALs)
  • A type of reading lens which decreases in power as we move towards the top of the lens
50
Q

What is Degressive/Occupational Lenses?

A
  • Designed based on near add AND reduction in plus power as you move more superior in the lens
51
Q

Advantages & disadvantages of Degressive/Occupational Lenses?

A
  1. ‘Soft’ design as surface astigmatism redistributed in the distance area
  2. Wearer obtains a wide central field with very low aberration
  3. Generally improves posture for the wearer, reducing neck & shoulder strain

Disadvantages:
1. NO DISTANCE – unsuitable for driving
2. Creates ‘artificial far point’ – closer than optical infinity

52
Q

What are Anti-Fatigue Lenses and who are they suitable for?

A
  • Designed as a distance vision lens with low ‘add’

Suitable for;
- Emerging presbyopes
- Patients with digital eye strain
- Emmetropic patients who require ‘accommodative’

Clinical Optics and Dispensing (10 decks)

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